Hi John, glad you made the right choice!
Ok now the sleeves are up and we can get down to business. First off, you're going to get mixed opinions as to whether 18mm ply is good enough to tile on. I sit firmly in the "it's not thick enough" group because I feel that, unless
very well supported, 18mm just isn't rigid enough to support tiles reliably.
Assuming you agree with this, you're going to need to do some work on your floor. There're two options I would recommend to you here:
Option 1 - You take up the current floor and replace with 25mm WBP ply, screwed down every 300mm to the joists. Don't forget to stagger the joints and make sure that you add extra support (noggins) under sheet edges between the joists. Then tile using Bal single part flexible adhesive and Bal superflex grout.
Option 2 - You ensure that the existing ply is screwed to the joists every 300mm. You then overboard with 9mm (use 12mm if your existing floor is springy) WBP ply which needs to be screwed down every 200mm all over the face of the sheets. Again, stagger the joints. Then tile using Bal single part flexible adhesive and Bal superflex grout.
Now option 2 is fine and will work no problem, but if you are prepared to replace the floor as in option 1 then that would result in a bomb proof job.
You mentioned in a previous post about needing to stick ply to tiles to bring the height up to match the rest of the floor - now you're tiling, you want to be sticking the ply down with the tile adhesive mentioned above and not gripfill.
A word on adhesives..... the adhesive I recommended is expensive but is vital - don't be tempted to save a few quid on a cheaper brand. It pays to search around for a good price - have a look online.
Good luck.